The measurement of total concentration of sulfuric acid appearing in the atmosphere as an aerosol would be relatively easy if the sulfuric acid appeared by itself, since there are a number of ways of measuring low concentrations of sulfur in a sample stream. For instance, the flame photometric detector (FPD) is an instrument which is very sensitive when performing this type of measurement. However, as a practical matter when measuring atmospheric content of sulfuric acid the problem is made much more difficult by the presence of interferants in the sample stream, often appearing as sulfur dioxide and ammonium sulfates, etc. Since a flame photometric detector measures the total sulfur content in a sample stream, the presence of other compounds of sulfur provides incorrect readings. It would be a solution to the measurement problem if there existed an instrument sensitive only to sulfuric acid, and not to these other species as well, but no such instrument is known at the present time. The problem is further complicated by the fact that sulfuric acid in aerosol suspension is relatively unstable, and tends to react with other entrained species. This fact makes it highly desirable to actually analyze the sample air stream at the site, and immediately after it is taken, rather than attempting to collect a sample for later laboratory analysis.
This approach necessitates the provision of a method and apparatus capable of performing sophisticated sample handling steps for the purpose of isolating the sulfuric acid from interfering species so that it can be promptly transferred to the flame photometric detector for analysis at the sampling site. The present invention seeks to eliminate certain interfering species, while at the same time collecting the sulfuric acid droplets on the walls of a collection chamber, the interfering species being flushed through the chamber, and the apparatus subsequently revaporizing the sulfuric acid and delivering it to the flame photometric detector.
There are several prior art methods for determining sulfuric acid content in ambient air. For instance, one method includes exposing filters to large quantities of air for a protracted period of time to collect sulfuric acid which can then be analyzed when taken from the filter. One method and apparatus of this general type is discussed in an article entitled "Collection of Sulfuric Acid Mist In the Presence of a Higher Sulfur Dioxide Background", by James V. Kerrigan and Karl Snajberg and Edwin S. Anderson, the article appearing in Analytical Chemistry, volume 32, No. 9, August 1960 beginning at page 1168.
Another prior art method and apparatus is discussed in an article entitled "Determination of Atmospheric Concentrations of Sulfuric Acid Aerosol by Spectrophotometry, Coulometry and Flame Photometry" by Scaringelli and Rehme, appearing in Analytical Chemistry, volume 41, No. 6, May 1969 beginning at page 707. This article recognizes that there are interferants such as sulfur dioxide and sulfates of ammonia which are present, but the article seeks to solve the problem by chemically changing the sulfuric acid into other sulfur compounds which are then measured by the use of one of the well-known detection approaches such as spectrophotometry, coulometry or flame photometry. Therefore, the method and apparatus according to this article does not perform sequential steps designed to eliminate these interferants and then directly measure the level of sulfuric acid as is done in the present disclosure.
U.S. Pat. No. 3,948,602 to Solomon teaches the method of monitoring in air the level of chloromethyl ether which includes the use of a tube for collecting certain compounds, the tube then being heated to volatilize the compounds so that they can be transported in a carrier gas to a hydrogen flame detector. This apparatus has two cycles including a sampling cycle and an analysis cycle. This patent and U.S. Pat. No. 3,550,429 both use timers to determine the duration of cycles in the measuring process.